Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station to which the client terminal is communicating with is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One of the commonly used duplexing methods is Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the Radio Frequency (RF) channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL while the RF channel may be used for UL during the second half. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically.
Yet another commonly used duplexing method is Half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
In many wireless communication systems, normally the communication between the base station and client terminals is organized into frames as shown in FIG. 3. The frame duration may be different for different communication systems and normally it may be in the order of milliseconds (ms). For a given communication system the frame duration may be fixed. In a TDD wireless communication system, a frame may be divided into a DL subframe and a UL subframe such that the communication from base station to the client terminal (DL) direction takes place during the DL subframe and the communication from client terminal to network (UL) direction takes place during UL subframe on the same RF channel.
Orthogonal Frequency Division Multiplexing (OFDM) systems typically use Cyclic Prefix (CP) to combat inter-symbol interference and to maintain the subcarriers orthogonal to each other under multipath fading propagation environment. The CP is a portion of the sample data that is copied from the tail part of an OFDM symbol to the beginning of the OFDM symbol as shown in FIG. 4. One or more OFDM symbols in sequence as shown in FIG. 4 are referred herein as OFDM signal.
Most wireless communication systems may employ some form of framing in the air interface. For example, 10 ms radio frames are used in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system and each radio frame comprises 10 subframes as shown in FIG. 5. Each subframe in turn consists of two slots and each slot consists of 6 or 7 OFDM symbols depending on the type of CP used as shown in FIG. 5. In 3GPP LTE wireless communication system, two different CP lengths are used and they are referred to as Normal CP and Extended CP. In wireless communication systems, normally the specific air interface frame structure repeats itself over certain periodicity.
In cellular wireless communication systems, the same frequencies may be used at the same time by base stations in neighboring cells. Therefore, performance of cellular wireless communication systems in many cases is limited by interference from the neighboring cells. The interference may occur both in the downlink direction and in the uplink direction. In interference limited cellular wireless communication systems, mainly two types of interference may need to be taken into consideration namely intra-cell interference and inter-cell interference. In intra-cell interference, the source of interference is in the same cell. This could occur, for example, when multiple client terminals are scheduled to receive or transmit on the same frequency resources at the same time. The intra-cell interference may also occur due to leakage from transmission in adjacent channels within a cell. In inter-cell interference, the source of interference is one or more adjacent cells. It is primarily caused by the use of same frequency channel in neighbor cells.
In 3GPP LTE, the smallest unit of radio resource that can be allocated to a user for data transmission is called Resource Block (RB). An RB is a time-frequency radio resource that spans over a time slot of 0.5 ms in the time domain and 12 subcarriers of 15 kHz bandwidth each in frequency domain with a total RB bandwidth of 180 kHz. The RB pairs over two consecutive timeslots in time domain may be allocated to a client terminal for data transmission in a Transmission Time Interval (TTI) of 1 ms known as a subframe as illustrated in FIG. 5. Orthogonal Frequency Division Multiplexing Access (OFDMA) is used in 3GPP LTE where the subcarriers are mutually orthogonal to each other, implying that there is no intra-cell interference. In 3GPP LTE, inter-cell interference can be a predominant factor limiting system performance especially for the client terminals located at cell edge areas. Inter-cell interference is caused as a result of collisions between RBs that are utilized by multiple adjacent cells simultaneously. When a client terminal moves away from the serving cell and becomes closer to a neighbor cell, the received signal quality may degrade as the received desired signal power may decrease and the received inter-cell interfering signal power may increase. In a multi-cell environment inter-cell interference can occur even when a client terminal is not in the cell edge areas. To handle large increase of data traffic in cellular wireless communication systems, cell density may be increased by adding more cells in a particular area but that in turn may result in increased inter-cell interference. In addition, introduction of Heterogeneous Networks (HetNet) with small cells inside macrocells using the same frequency, may increase inter-cell interference even further.
Multiple transmit and/or receive chains are commonly used in many wireless communication systems for different purposes. Using multiple transmit and/or receive chains, the spatial dimension can be exploited in the design of a wireless communication system. Wireless communication systems with multiple transmit and/or receive chains offer improved performance. Different techniques using multiple transmit and/or receive chains are often referred to with different terms such as Maximal Ratio Combining (MRC), Space-Time Coding (STC) or Space-Time Block Coding (STBC), Spatial Multiplexing (SM), Beam-Forming (BF) and Multiple Input Multiple Output (MIMO). Wireless communication systems with multiple transmit chains at the transmit entity and multiple receive chains at the receive entity are generically referred as MIMO systems. For client terminals in inter-cell interference scenarios, a Minimum Mean Square Error-Interference Rejection Combining (MMSE-IRC) receiver may be used to effectively improve the throughput performance.
The conventional MMSE receiver may process a received signal assuming that the statistical characteristics and the impact of interference from neighbor cells may be similar to that of noise received by a client terminal. Thus, in environments where the power level of the interference signal is higher than that of the noise, inter-cell interference may degrade the client terminal throughput. Using an MMSE-IRC receiver as a client terminal interference rejection and suppression technology to mitigate the effects of inter-cell interference, increases client terminal throughput even in scenarios where high interference is experienced by the client terminal. The MMSE-IRC receivers may be able to use multiple receiver antennas to create nulls in the arrival direction of an interference signal, by reducing the antenna gain which in turn may suppress the interfering signal, thereby improving the Signal to Interference and Noise Ratio (SINR) and throughput.
For achieving the throughput performance improvement, the MMSE-IRC receiver requires the knowledge of the statistics of the interference signals, such as a covariance matrix. A covariance matrix is a matrix whose diagonal components express the variance of each variable in a set of variables and each of the off-diagonal elements express the degree of correlation between two variables with respect to their direction of change. As the covariance matrix may not be known a priori, it may need to be estimated from the received signal. For example, the covariance matrix may be estimated from the composite signal which includes the interference signals and the desired signal. The covariance matrix estimation requires knowledge of propagation channel for the desired signal. This in turn makes the performance of MMSE-IRC receiver sensitive to the channel estimation errors as well as covariance matrix estimation errors.
Since a MMSE-IRC receiver is sensitive to covariance matrix estimation errors, by improving the accuracy of covariance matrix estimation, the throughput performance of the MMSE-IRC receiver may be significantly improved. In wireless communication systems, a client terminal may experience different types of fading conditions with different frequency selectivity. For example, in case of a 3GPP LTE wireless communication system, fading profiles such as Extended Pedestrian A (EPA), Extended Vehicular A model (EVA) and Extended Typical Urban model (ETU) are defined to address the different types of a propagation environment a client terminal may experience. Furthermore, a client terminal may experience different Signal-to-Noise Ratio (SNR) and SINR conditions. The throughput performance of the MMSE-IRC receiver may vary depending on the fading and geometry conditions. Here geometry is defined to be the same as that of the SINR. Specifically, the geometry factor G is defined as follows:
  G  =            Signal      ⁢                          ⁢      power      ⁢                          ⁢      of      ⁢                          ⁢      serving      ⁢                          ⁢      cell                      Interference        ⁢                                  ⁢        power            +              Noise        ⁢                                  ⁢        power            
In 3GPP LTE wireless communication system, the signal conditions may vary depending on the number of client terminals in a cell, SINR at different client terminals, the location of all the active client terminals, etc.